netdev - Re: "lockless" qdisc breaks tx_queue

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Date:   Wed, 3 Jan 2018 10:09:22 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: "lockless" qdisc breaks tx_queue_len change too?

On 01/02/2018 08:41 PM, Cong Wang wrote:
> Hi, John
> 
> While reviewing your ptr_ring fix again today, it looks like your
> "lockless" qdisc patchset breaks dev->tx_queue_len behavior.
> 
> Before your patchset, dev->tx_queue_len is merely an integer to read,
> after your patchset, the skb array has to be resized when
> dev->tx_queue_len changes, but I don't see any qdisc code handles
> this...
> 
> Also, because of that, I doubt __skb_array_empty() in
> pfifo_fast_dequeue() can be safe any more even with your ptr_ring fix.
> 
> What am I missing?
> 

I dropped support for tx_queue_len changes after qdisc has been
created. The only check is at init time when building the qdisc.

Before this series teql and pfifo_fast were the only qdiscs that
used tx_queue_len other qdiscs used other mechanisms or copied
tx_queue_len at init time. So the API is inconsistent.

OK, but arguably its kAPI now and needs to be supported on live
qdiscs. So couple options drop the __skb_array_empty() check,
stop supporting changes on running qdiscs, or do a qdisc swap
with the new array.

I'm tempted to make the qdisc swap work, still need benchmarks
I guess without the empty check. Either way to get it working
we need a callback from tx_queue_len code paths.

Unfortunately, I guess someone somewhere probably uses pfifo_fast
and changes there queue length with a script after creating the
qdisc and expects it to work.

> Thanks.
>